1. Field of Invention
The present invention relates generally to the art of magnetic tunnel junction (MTJ) read head devices, which sense magnetic fields in a magnetic recording medium. More particularly, the present invention relates to a magnetic tunnel junction arrangement having a tunneling barrier made of particular materials that result in high tunneling performance. The invention finds particular application in conjunction with reading hard disk drives and will be described with particular reference thereto. However, it is to be appreciated that the invention will find application with other magnetic storage media. Further, it is to be appreciated that the invention will find application in other magnetic field detection devices as well as in other devices and environments.
2. Description of the Related Art
Magneto-resistive (MR) sensors based on anisotropic magneto-resistance (AMR) or a spin-valve (SV) effect are widely known and extensively used as read transducers to read magnetic recording media. Such MR sensors can probe the magnetic stray field coming out of transitions recorded on a recording medium by generating resistance changes in a reading portion formed of magnetic materials. AMR sensors have a low resistance change ratio or magneto-resistive ratio xcex94R/R, typically from 1 to 3%, whereas SV sensors have a xcex94R/R ranging from 2 to 7% for the same magnetic field excursion. SV heads showing such high sensitivity are able to achieve very high recording densities, that is, over several giga bits per square inch or Gbits/in2. Consequently, SV magnetic read heads are progressively supplanting AMR read heads.
In a basic SV sensor, two ferromagnetic layers are separated by a non-magnetic layer, an example of which is described in U.S. Pat. No. 5,159,513. An exchange or pinning layer of FeMn, for example, is further provided adjacent to one of the ferromagnetic layers. The exchange layer and the adjacent ferromagnetic layer are exchange-coupled so that the magnetization of the ferromagnetic layer is strongly pinned or fixed in one direction. The magnetization of the other ferromagnetic layer is free to rotate in response to a small external magnetic field. When the magnetizations of the ferromagnetic layers are changed from a parallel to an anti-parallel configuration, the sensor resistance increases yielding a relatively high MR ratio.
Recently, new MR sensors using tunneling magneto-resistance (TMR) have shown great promise for their application to ultra-high density recordings. These sensors, which are known as magnetic tunnel junction (MTJ) sensors or magneto-resistive tunnel junctions (MRTJ), came to the fore when large TMR was first observed at room temperature. See Moodera et al, xe2x80x9cLarge magneto resistance at room temperature in ferromagnetic thin film tunnel junctions,xe2x80x9d Phys. Rev. Lett. v. 74, pp. 3273-3276 (1995). Like SV sensors, MTJ sensors basically consist of two ferromagnetic layers separated by a non-magnetic layer. One of the magnetic layers has its magnetic moment fixed along one direction, i.e., the fixed or pinned layer, while the other layer, i.e., free or sensing layer, is free to rotate in an external magnetic field. However, unlike SV sensors, this non-magnetic layer between the two ferromagnetic layers in MTJ sensors is a thin insulating barrier or tunnel barrier layer. The insulating layer is thin enough so that electrons can tunnel through the insulating layer. Further, unlike SV sensors, MTJ sensors operate in CPP (Current Perpendicular to the Plane) geometry, which means its sensing current flows in a thickness direction of a laminate film or orthogonal to the surfaces of the ferromagnetic layers.
The sense current flowing through the tunnel barrier layer is strongly dependent upon a spin-polarization state of the two ferromagnetic layers. When the sense current experiences the first ferromagnetic layer, the electrons are spin polarized. If the magnetizations of the two ferromagnetic layers are anti-parallel to each other, the probability of the electrons tunneling through the tunnel barrier is lowered, so that a high junction resistance Rap is obtained. On the other hand, if the magnetizations of the two ferromagnetic layers are parallel to each other, the probability of the electrons tunneling is increased and a high tunnel current and low junction resistance Rp is obtained. In an intermediate state between the parallel and anti-parallel states, such as when the both ferromagnetic layers are perpendicular in magnetization to each other, a junction resistance Rm between Rap and Rp is obtained such that Rap greater than Rm greater than Rp. Using these symbols, the TMR ratio may be defined as xcex94R/R=(Rapxe2x88x92Rp)/Rp.
The relative magnetic direction orientation or angle of the two magnetic layers is affected by an external magnetic field such as the transitions in a magnetic recording medium. This affects the MTJ resistance and thus the voltage of the sensing current or output voltage. By detecting the change in resistance and thus voltage based on the change in relative magnetization angle, changes in an external magnetic field are detected. In this manner, MTJ sensors are able to read magnetic recording media.
Another problem is a trade-off between high TMR ratio and MTJ resistance. The TMR ratio is proportional to the spin polarization of the two ferromagnetic layers. A TMR ratio as high as 40% was achieved by choosing a preferable composition for the two ferromagnetic layers. See Parkin et al., xe2x80x9cExchange-biased magnetic tunnel junctions and application to nonvolatile magnetic random access memory,xe2x80x9d J. Appl. Phys., v. 85, pp. 5828-5833 (Apr. 15, 1999). However, despite this large TMR ratio, the application of such MTJs in read heads was, up to now, prohibitory due to the large resistance of the junctions, resulting in a large shot noise Vrms and a poor signal to noise ratio S/N. Shot noise Vrms=(2xc2x7exc2x7Ixc2x7xcex94f)xc2xdxc3x97R, where: e=1.6xc3x9710xe2x88x9219C; I=current; xcex94f=bandwith; and R=junction resistance.
It is possible to reduce the MTJ""s resistance-area product Rxc2x7A or RA using a natural, in situ oxidation method. RA is a characteristic of an insulating barrier and contributes to junction resistance R through the equation R=Rxc2x7A/junction area. Using a 7 xc3x85 or less Al layer that is properly oxidized, an RA as low as 15 xcexa9.xcexcm2 has been achieved. This remarkably low value together with the high TMR ratio make MTJs very attractive for application as read heads for very high recording densities.
However, yet another problem in MTJs is that the thin insulating barrier is very sensitive to one of the manufacturing processes called lapping. Lapping involves the definition of an air bearing surface (ABS) on the MTJ head. Because the insulating barrier is so thin, lapping can create electrical shorts between the two adjacent magnetic layers, rendering the sensor useless.
Tunneling magnetoresistance (TMR) was discussed by Julliere in xe2x80x9cTunneling Between Ferromagnetic Filmsxe2x80x9d Physics Letters, 54A 225 (1975). However, prior to 1995, the reported MTJ junctions only show very small TMR response at room temperature, at best being on the order of 1-2%.
An MTJ device with a large TMR over 10% at room temperature was reported by Moodera et al. in the aforementioned article xe2x80x9cLarge Magnetoresistance at Room Temperature in Ferromagnetic Thin Film tunnel Junctionsxe2x80x9d Physics Review Letters, 74, 3273 (1995). It was hypothesized that increased TMR performance could be achieved by a decrease in surface roughness that results from the base electrode growth, by evaporation onto a cryogenically-cooled substrate, by the use of a seed layer, and by keeping the base electrode extremely thin. The tunnel barrier was formed by cryogenically depositing an Al layer and subsequently warming this layer and plasma oxidizing it to consume more of the Al. The resulting junction resistances were in the range of hundreds of Ohms to tens of kxcexa9 for junctions with cross-sectional areas of 200xc3x97300 xcexcm2.
The relatively large junction resistance of some MTJ device arrangements severely limits their use in particular applications, such as read head applications for example, due to the low signal to noise ratio (S/N) that results from their relatively high junction resistance values. While some of these MTJ arrangements may have favorable TMR response values, their corresponding low signal to noise ratios diminish the advantage provided by their TMR values. The junction resistance factor becomes even more critical as the junction resistance is scaled up when junction size is decreased, as is required for high area density recording applications. Accordingly, a need remains for an MTJ device arrangement having a sufficiently large TMR response at room temperature, while still providing a reasonably low junction resistance.
Therefore, a goal of the present invention is to provide a MTJ read head design in which the resulting TMR ratio is maximized by choosing particular tunneling barrier materials for the MTJ. These particular tunneling barrier materials should provide a reasonably low junction resistance while still maintaining a high performance TMR response.
Another goal of the present invention is to provide a design wherein the tunneling barrier in MTJ has relatively large thickness while still maintaining a reasonably low junction resistance by choosing particular tunneling barrier materials with low barrier height.
Accordingly, the present invention is directed to a magnetic tunnel junction device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
A magnetic tunnel junction (MTJ) is made up of two ferromagnetic layers, one of which has its magnetic moment fixed and the other of which has its magnetic moment free to rotate. Located between these two ferromagnetic layers is an insulating tunneling barrier layer for permitting tunneling current to flow perpendicularly through the layers. The insulating barrier is preferably formed by oxidation of a thin metallic alloy layer.
One advantage of the present invention is that it provides an MTJ having a nonmagnetic tunneling barrier with a relatively low barrier height. By forming the MTJ with insulating materials that result in a low barrier height, it is possible to fabricate the MTJ with a relatively thick insulating barrier while still maintaining low junction resistance, as desired for magnetoresistance read head applications.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments taken together with the accompanying figures.